When one is in a relationship that both people know is ending and one of the two people starts to look for another relationship. This could be described as cheating but is usually not recognized as such as both people know that the end of the current relationship is emminent.

"Did I see you with Carrie yesterday?"
"Yup"
"Dude, I thought you were still dating Melissa?"
"I am but it's pretty much over and I'm overlapping"